Photometry of Lights of Different Colours. 179 



effects are Jess by it, would support such a belief. The 

 existence of a reversed Purkinje effect and a reversed yellow- 

 spot effect, as found in the present investigation, appear to 

 render Dow's theory inadequate. While a shift from cone 

 to rod action explains very well the known facts of the 

 Purkinje and yellow-spot effects and fits with the distribu- 

 tion of rods and cones in the retina, the flicker phenomena 

 would appear to call for a more complicated mechanism of 

 seeing. 



Up to the present, the various facts upon which a more 

 complete hypothesis may be formed are : — 1st, The large in- 

 crease in blue sensitiveness at low illuminations for large 

 fields, and the smaller increase for small fields, by the equality 

 of brightness method. 2nd, The large increase of red sensi- 

 tiveness at low illuminations for large fields, and the smaller 

 increase for small fields, by the flicker method. 3rd, The 

 greater sensitiveness of the flicker method at high than at 

 low illuminations, and for central as compared with peri- 

 pheral vision. 4th, The complete failure of the flicker method 

 at very low illuminations. 5th, The higher speed necessary 

 for a large field centrally viewed as compared with a small 

 field, and as compared with a small peripheral field. 



Certain of these facts seem to support Dow's contention 

 that the cones are probably the chief seat of action with the 

 flicker photometer. The failure of the flicker method at 

 low illuminations, where the rods are coming into play, 

 speaks particularly for this. But it is necessary to ascribe 

 some complexity of structure and behaviour- to the cones if 

 they are to suffice. As has been established by study of the 

 retina, the cones are scarcer towards the periphery, but at 

 the same time change in character, becoming larger. If we 

 ascribe to these peripheral cones the property of exhibiting 

 the typical cone characteristics (red sensitiveness and sensi- 

 bility to flicker) more decidedly as the illumination decreases, 

 we can account for some of the phenomena described above. 

 In equality of brightness measurements such greater red 

 sensitiveness in the cones would tend to compensate for the 

 increasing activity of the rods and to preserve conditions of 

 vision constant. If the flicker method uses only the cones, 

 decreased illumination would bring out this compensating 

 action at illuminations where the equality of brightness 

 method would show comparatively little Purkinje effect, both 

 retinal elements still being in action. As a matter of fact the 

 curves of Plate iv. (PI. V.) for five observers at an illumi- 

 nation of 10 units do exhibit, on the whole, a more marked 

 shift toward red by the flicker method than is the shift toward 



N2 



